Step 1: Parts and Tools (cont..)
USB Charging Circuit
USB charging requires 5V at around 500mA of power to charge most gadgets. In order to meet these criteria in a small package we use a USB Charging Circuit, which
boosts low voltage (2V) DC up to 5V DC. Nothing comes free though; we trade our increased volts for fewer amps. To avoid this tradeoff, we could use larger solar
panels and larger batteries, but this would result in a larger, heavier, and more expensive charger.
If you're using an older Apple product, make sure your USB Charging Circuit is compatible. Typically a USB gadget requires just power from a USB port to charge.
Most iPhone models, however, require a tweak to USB to work. iPhones will check the USB data ports in order to identify what kind of device it's plugged into and
change charging speeds if it thinks the port can support it. Unfortunately if it doesn't see anything on the data tabs it will refuse to charge. This is a problem that
plagues many generic USB chargers, especially older ones.
Our USB Charging Circuit needs a minimum of 2V to operate, so we need to choose power sources with this in mind.
Battery Power
We always advise the use of power storage when doing a solar project. Solar is inconsistent due to the inconsistent nature of the sun. Using batteries helps stabilize the
flow of power, and also lets us hold onto that energy for later use.
In this project we use three rechargeable AA batteries. Rechargeable batteries put out 1.2V of power and using three in series gives us 3.6V. Using two batteries will
cause our voltage to drop below 2V too quickly, and our USB Charging Circuit won’t be able to operate. Four batteries would require a much larger solar cell to charge.
Solar Power
Batteries require a minimum voltage in order to charge, but raising the voltage will not cause the batteries to charge faster. The general rule is to provide 1.5 times as
much voltage as your battery needs (for us 3.6V x 1.5V = 5.4V), but with solar we want some wiggle room. We want our solar panel to meet the minimum voltage, even
on days with a few clouds, so a 6V panel does well.
When charging AAs using a wall adapter, we're able to charge them at high speed due to smart chips that constantly monitor the battery. Since our circuit is "dumb," with
nothing to monitor the batteries, we trickle charge the batteries using the 10% rule. AAs can be safely charged as long as they don't have more than 10% of their capacity
thrown at them at any one time. This means our 2,000mAh batteries should only get 200mA of current.
Our solar cell is rated for only 80mA, so we're completely safe! Different panels and batteries vary, so make sure yours work well together. One good aspect of the trickle
charge method is that it will never over-charge the batteries. Once they're full they just stay topped off.
Circuit Diagram
Below is a circuit diagram for this project. It's designed so that the solar cell is always charging the batteries and the toggle switch turns the USB circuit on/off. A diode
has been put in place to prevent power from flowing the wrong direction into the solar cell. This is a very common diagram for solar. This basic setup can easily be scaled
up or down depending on the project.
